In recent years, iPS cells of mouse and human have been established. Yamanaka et al. established iPS cells by introduction of Oct3/4, Sox2, Klf4 and c-Myc genes into mouse-derived fibroblasts and their forced expression therein (1, 2). Thereafter, it was found that iPS cells can be prepared with 3 factors, without the c-Myc gene (3). Further, Yamanaka et al. succeeded in establishment of iPS cells by introduction of 4 human genes which correspond to the mouse genes into skin-derived fibroblasts of human (1, 4). On the other hand, Thomson et al. prepared human iPS cells by using Nanog and Lin28 instead of Klf4 and c-Myc (5, 6).
Virus vectors such as retroviruses and lentiviruses are excellent vectors in view of their higher gene transfer efficiencies as compared with non-viral vectors, which enable simple preparation of iPS cells using them. However, retroviruses and lentiviruses are incorporated into chromosomes. Similarly, even with a plasmid vector which is generally considered to be less prone to be incorporated into chromosomes, a stably expressing cell line in which the reprogramming genes are incorporated into chromosomes is obtained at a certain frequency, which may be due to requirement of continuous high expression of the reprogramming factors in establishment of the iPS cells (7, 8).
On the other hand, after establishment of iPS cells, there are cases where repression of expression (or silencing) of the incorporated exogenous genes occurs. Such iPS cells in which expression of the incorporated exogenous genes is repressed are preferably used for clinical application, and so on.
Thus, a method for assaying whether expression of exogenous transgenes is repressed and a method for selecting an iPS cell by assessing the gene repression are demanded.